1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, to a pixel, an organic light emitting display including the same, and a driving method thereof, in which a gradation is represented using the frequency characteristic of an organic light emitting diode.
2. Discussion of Related Art
Recently, various flat panel displays have been developed as alternatives to a relatively heavy and bulky cathode ray tube (CRT) display. The flat panel display includes a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), a light emitting diode (LED) display, etc.
Among the flat panel displays, the light emitting diode display can emit light for itself by electron-hole recombination to allow a fluorescent layer thereof to emit the light. The light emitting diode display can be classified into an inorganic light emitting diode display and an organic light emitting diode (OLED) display according to materials and structures thereof. Here, the organic light emitting diode display can also be referred to as an organic light emitting display or an electroluminescent display.
Unlike the liquid crystal display (LCD) requiring a separate light source, an organic light emitting display has an advantage of fast response time like the CRT display.
FIG. 1 is a circuit diagram of a pixel provided in a conventional organic light emitting display.
Referring to FIG. 1, a pixel 11 of a conventional organic light emitting display is disposed in a region where a scan line Sn intersects (or crosses) a data line Dm. The pixel 11 is selected when a scan signal is applied to the scan line Sn, and emits light based on a data signal applied to the data line Dm.
The pixel 11 is connected to a first power source VDD and a second power source VSS, and includes an organic light emitting diode OLED and a pixel circuit 40.
The organic light emitting diode OLED includes an anode electrode connected to the pixel circuit 40, and a cathode electrode connected to the second power source VSS.
The organic light emitting diode OLED includes an emitting layer, an electron transport layer, and a hole transport layer, which are interposed between the anode electrode and the cathode electrode. Additionally, the organic light emitting diode OLED may include an electron injection layer, and a hole injection layer. In this organic light emitting diode OLED, when a voltage is applied between the anode electrode and the cathode electrode, electrons emitted from the cathode electrode are moved to the emitting layer via the electron injection layer and the electron transport layer, and holes generated from the anode electrode are moved to the emitting layer via the hole injection layer and the hole transport layer. Then, the electrons from the electron transport layer and the holes from the hole transport layer are collided and recombined with each other in the emitting layer, thereby emitting the light.
The pixel circuit 40 includes a first transistor M1, a second transistor M2, and a capacitor C. Here, each of the second transistor M2 and the first transistor M1 includes a p-type metal oxide semiconductor field effect transistor (MOSFET). Also, the second power source VSS has a lower voltage level than the first power source VDD. For example, the second power source VSS has a ground voltage level.
The first transistor M1 includes a gate electrode connected to the scan line Sn, a source electrode connected to the data line Dm, a drain electrode connected to a first node N1. Here, the first transistor M1 supplies the data signal from the data line Dm to the first node N1 in response to the scan signal supplied to the scan line Sn.
The capacitor C stores a voltage corresponding to the data signal applied to the first node N1 via the first transistor M1 while the scan signal is supplied to the scan line Sn, and then keeps the second transistor M2 turned on when the first transistor M1 is turned off.
The second transistor M2 includes a gate electrode connected to the first node N1 to which the drain electrode of the first transistor M1 and the capacitor C are connected in common, a source electrode connected to the first power source VDD, and a drain electrode connected to the anode electrode of the organic light emitting diode OLED. Here, the second transistor M2 adjusts the amount of current in correspondence to the data signal supplied from the data line Dm and applied to the organic light emitting diode OLED. Thus, the organic light emitting diode OLED emits light based on the current supplied from the first power source VDD via the second transistor M2.
The pixel 11 operates as follows. First, while the scan signal of a low state is applied to the scan line Sn, the first transistor M1 is turned on. Then, the data signal is supplied from the data line Dm to the gate electrode of the second transistor M2 via the first transistor M1 and the first node N1. At this time, the capacitor C stores a voltage corresponding to the voltage difference between the gate electrode of the second transistor M2 and the first power source VDD.
Thus, the second transistor M2 is turned on by the voltage applied to the first node N1, and supplies the current corresponding to the data signal to the organic light emitting diode OLED. Hence, the organic light emitting diode OLED emits light based on the current applied from the second transistor M2, thereby displaying an image.
Then, while the scan signal of a high state is applied to the scan line Sn, the second transistor M2 is kept being turned on by the voltage corresponding to the data signal stored in the capacitor C, so that the organic light emitting diode OLED emits light and displays an image in one frame.
Further, a conventional organic light emitting display additionally may include a compensation circuit (not shown) to compensate for the non-uniformity of the threshold voltages of a plurality of second transistors (e.g., the second transistor M2) due to a manufacturing process. However, although the conventional organic light emitting display may include the compensation circuit to operate in an offset compensation manner or a current programming manner, there is still a limit to display an image with uniform brightness.